The invention relates to a method for operating a multi-phase modular multi-level power converter having a load connected to the AC voltage terminals thereof.
The topology of a modular multi-level power converter is known from DE 101 03 031A1. In the literature, the modular multi-level power converter is also termed an M2C or also an M2LC (Modular MultiLevel Converter). Like a cascaded H-bridge converter, this modular multi-level power converter is classed as a cell converter. In the publication entitled “New Concept for High Voltage—Modular Multilevel Converter”, by R. Marquardt and A. Lesnicar, published for the PESC 2004 Conference in Aachen, the main types of multi-level power converter are analyzed in respect of industrial implementation and compared with one another, wherein the use of a modular multi-level power converter in a back-to-back intertie application is also proposed.
In contrast to a cascaded H-bridge converter, the power converter cells of a modular multi-level power converter as disclosed in DE 101 03 031A1 each have a half-bridge comprised of two series-connected turn-off semiconductor switches, in particular Insulated-Gate Bipolar Transistors (IGBTs), wherein an energy store implemented as a DC capacitor is provided for the storage of energy.
According to the older national patent application with official application number 10 2011 006 988.7, such a DC capacitor comprises a large number of commercially available capacitors, in particular electrolytic capacitors, which are connected in parallel and/or in series. Instead of electrolytic capacitors, film capacitors can also be used. This high capacitor count results in an increased space requirement for a power converter having a large number of converter cells, causing increased design and mechanical complexity.
According to DE 101 03 031A1, each phase module of a modular multi-level power converter has a large number of series-connected power converter cells which are also termed submodules. It is advantageous if the power converter cells of a phase module are symmetrically distributed over its two valve arms. In contrast to the cascaded H-bridge converter, the submodule capacitors are not charged by separate diode rectifiers, but via the connection to the DC link. Due to the connection of the electrically series-connected submodules of a phase module, e.g. of a three-phase modular multi-level power converter on the DC bus, for symmetrical operation within the six valve arms a module voltage according to the following equation:
                                          u            _                                Kli            ,            j                          =                                            1                              T                0                                      ⁢                                          ∫                0                                  T                  0                                            ⁢                                                u                                      Kli                    ,                    j                                                  ⁢                                  ⅆ                  t                                                              =                                                    u                d                                            2                ⁢                N                                      ≈                                          U                d                                            2                ⁢                N                                                                        (        1        )            where i=1, 2, . . . 6 and j=1, 2, . . . N must be present, averaged over time, at the terminals of each submodule. For typical applications of a modular multi-level power converter, the load current has a well-pronounced fundamental component. As the load current flows proportionately through the modules within a phase module, it follows from the product of terminal voltage uKlij and valve arm current iZi according to the following equationpKli,j=uKli,jizi  (2)and equation (1) that energy variations in the fundamental frequency are very pronounced in the capacitors. In particular these fundamental frequency components of the energy are extremely important for dimensioning the capacitors. The fundamental frequency voltage components make it necessary, among other things, to install high-value capacitors. These high capacitance values are achieved by connecting a plurality of capacitors in parallel. This high capacitor count, described for a two-pad power converter cell of the already mentioned national patent application with official application number 10 2011 006 988.7, leads to an increased power converter space requirement, resulting in a high degree of design and mechanical complexity and increased safety requirements.
Important figures for comparing different voltage converter topologies are the energy WC,ges capacitively stored in the converter, referred to the maximum apparent power SMax (SMax can also be an operand), according to the equation:
                              k          JproVA                =                              W                          C              ,              ges                                            S                          Ma              ⁢                                                          ⁢              x                                                          (        3        )            and the energy WCMax,ges capacitively storable in the power converter according to the equation:
                              k                      JproVA            ,                          Ma              ⁢                                                          ⁢              x                                      =                              W                          CMax              ,              ges                                            S                          Ma              ⁢                                                          ⁢              x                                                          (        4        )            wherein energy WCMax,ges capacitively storable in the power converter is calculated using the following equation:
                              W                      CMax            ,            ges                          =                              ∑                          i              =              1                        6                    ⁢                                    ∑                              j                =                1                            N                        ⁢                                          1                2                            ⁢                              C                                  SMi                  ,                  j                                            ⁢                              U                                  N                                      Ci                    ,                    j                                                  2                                                                        (        5        )            where UNCi,j is the rated voltages of the capacitors.
For medium voltage applications, typical values for kJproVA (equation (3)) of a 3-level voltage converter are 6 . . . 9 kJ/MVA. In the case of the modular multi-level power converter, the values for kJproVA and therefore of the stored energy are likely to be considerably higher.
A high stored or storable energy in the power converter is disadvantageous both for cost reasons (capacitor costs, space requirement, . . . ) and on safety grounds. Both could limit possible fields of application for the modular multi-level power converter.
Reducing the capacitor cost/complexity of a modular multi-level power converter, particularly of a three-phase converter, could open up new fields of applications for the multi-level power converter.
The first question to be answered is how to dimension the capacitors of the power converter cells of, for example, a three-phase modular multi-level power converter.
Relevant to the dimensioning are requirements such as                permissible maximum/minimum voltage of the capacitors,        permissible losses in the capacitors, and possibly        permissible ripple voltage.        
A submodule energy wCi,j stored over a period T0 averaged over time is calculated according to the following equation:
                                          w            _                                Ci            ,            j                          =                              1                          T              0                                ⁢                                    ∫              0                              T                0                                      ⁢                                          1                2                            ⁢                              C                                  SMi                  ,                  j                                            ⁢                              u                                  Ci                  ,                  j                                2                            ⁢                              ⅆ                t                                                                        (        6        )            
A capacitively stored energy wC,ges of the power converter is determined according to the equation:
                              w                      C            ,            ges                          =                              ∑                          i              =              1                        6                    ⁢                                    ∑                              j                =                1                            N                        ⁢                                          1                2                            ⁢                              C                                  SMi                  ,                  j                                            ⁢                              u                                  Ci                  ,                  j                                2                                                                        (        7        )            
The publication “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen” (“Modular power converter concept for high-voltage grid intertie applications”) by Rainer Marquardt, Anton Lesnicar and Jürgen Hildinger, reproduced in the proceedings of the ETG Conference 2002, Bad Nauheim, April 2002, and “Control of the Modular Multi-Level Converter for Minimized Cell Capacitance”, by Stephan P. Engel and Rik W. De Doncker, reproduced in the conference proceedings of the 14th European Conference on Power Electronics and Applications (EPE 2011), Birmingham, UK, 30 Aug.-1 Sep. 2011, pp. 4351-4360, each present a control method whereby the energy stores of the power converter cells of a multi-level power converter can be minimized. In both of these publications, back-to-back intertie operation is quoted as an application.
In the publication “On Dynamics and Voltage Control of the Modular Multilevel Converter” by Antonios Antonopoulos, Lennart Ängquist and Hans-Peter Nee, reproduced in the proceedings of the 13th European Conference on Power Electronics and Applications (EPE 2009), Barcelona, Spain, 8-10 Sep. 2009, pp. 3353-3362, a control method for a modular multi-level power converter is presented with which a reduction in the average capacitor voltage—and therefore in the energy stored, averaged over time, as a function of the load voltage—is achieved. Cited as a positive effect are reduced switching losses and higher permissible voltage ripple in the capacitors.
The invention is based on the insight that the energy wCi,j stored in the capacitors has a time-dependent component {tilde over (w)}Ci,j and a constant component wCi,j at a steady-state operating point. The time-dependent component {tilde over (w)}Ci,j of the energy stored in the capacitor is determined by the operating mode of the modular multi-level power converter or more specifically of the load connected thereto. Influencing factors include the operating point of the load, the shape of the common-mode voltage as well as internal “circulating currents”.